This invention relates to a paste material used in a rechargeable, alkaline, zinc-based battery and more particularly this invention relates to a matrix for zinc particles in an anode for a zinc-silver secondary battery.
In modern electronics, there is a need for high performance, rechargeable batteries. A silver-zinc battery, having one of the highest energy densities both per unit weight and per unit volume plus extremely high discharge rates, appears to be an ideal solution to this need. However, a zinc-based battery has historically presented many challenges for the battery manufacturer. One of these challenges involves containing zincate diffusion within the battery. This is a particularly severe problem because zinc is extremely soluble in the strongly alkaline environments routinely used as the electrolyte in these types of batteries. The presence of KZn(OH)x where x=1 to 3 presents a double-edged sword for the battery designer. On the upside, the high solubility allows for rapid current spikes typically unattainable with other battery systems. However, this high solubility diffuses zinc ions into undesired locations within the battery. Upon re-plating, this zinc diffusion leads to the well-known phenomena of electrode shape change and the presence of zinc dendrites within the batteries. This shape change includes an agglomeration of the zinc towards the center of the battery while simultaneously depleting zinc from the edges. Dendrites can readily be formed due to the zinc concentration gradients within the battery. Their tree-like structures have as their most undesirable effect the rupture of the separator membranes leading to battery shorting.
Researchers in this area have tried various approaches to control the electrode shape change and to reduce the zinc dendrite formation. They can be classified into five different categories. The first approach involves taking into account the redistribution of zinc by starting out with zinc depleted at the center and agglomerated at the edges. The second approach attempts to deal with the issue by modifying the electric field experienced by the zinc so that the edges experience a stronger electric field than the center. A third approach involves attempts to decrease the solubility of the zinc by complexing it with other agents. The fourth approach involves attempts to contain the solubility of zinc by encapsulating it in a matrix, typically a matrix involving gelling agents. The fifth and final approach involves attempts to make a separator resistant to zinc dendrites. Encapsulation is the most promising of the approaches to improve anode performance.
Encapsulation has focused on using agents that swell easily in the presence of the electrolyte. In U.S. Pat. No. 5,686,204, Bennet et al use crosslinked CARBOPOL acrylic acid as a gelling agent possessing high absorbency. In U.S. Pat. No. 4,368,244 Danzig uses paste material composed of diacetone acrylamide and acrylic acid. Sehm in U.S. Pat. No. 4,778,737 teaches zinc surrounded by acrylamide and acrylate polymers.
Polyethylene oxide (PEO) has been disclosed in several patents as a gelling agent, including U.S. Pat. No. 5,384,214 by Sugihara et al who use a surfactant made of PEO and a perfluoroalkyl chain. Similarly, Getz et al in U.S. Pat. No. 5,464,709 use a methoxylated polyethylene oxide (Carbowax 550).
Still others have researched the use of crosslinked vinyl alcohols as preferred gelling agents. Thus, Ito et al in U.S. Pat. No. 5,525,444 disclose an electrode with a paste made from a vinyl alcohol crosslinked to a moiety containing COOX groups. Suga et al in U.S. Pat. No. 5,382,482 place zinc in direct contact with a crosslinked polymer, such as in the crosslinking of polyvinyl alcohol and dimethyldiethoxysilane.
Polyacrylic acid is used by Shinoda et al in U.S. Pat. No. 5,376,480 while Goldstein in U.S. Pat. No. 5,206,096 discloses a mixture of organic inhibitors and a gelling agent such as polyacrylic acid, carboxymethyl cellulose and hydrolyzed polyacrylonitrile. Kordesh et al in U.S. Pat. No. 5,281,497 teach crosslinked starch as the preferred gelling agent.
Finally, Suga et al in U.S. Pat. No. 5,348,820 suggest a polymer layer which is in contact with the zinc, said polymer having an oxygen permeability constant greater than 10xe2x88x9213 cm3 cmxe2x88x921sxe2x88x921Paxe2x88x921.
None of the above patents address certain peculiarities of zinc electrodes. First, there is the tendency of zinc to expel the gelling agent upon re-plating. This creates void spaces for zinc ion diffusion. Second, there are significant density changes when zinc is discharged to zinc oxide and vice versa. Finally, these patents do not address the hydrogen production that occurs during overcharge. All of these effects, acting separately and in concert, contribute to the breach of the initial zinc matrix.
The present invention provides a material that surrounds the zinc in a three-dimensional lattice matrix which induces the zinc to re-plate in the same mesh size as it was originally assembled. Second, the material has been designed to be mechanically stable despite zinc cycling. Third, the anode paste of the invention remains electrically interconnected during the entire charge cycle. Finally, the anode has high ionic transport, excellent accommodation to zinc density changes and, optionally, high hydrogen transport.
The material used as the anode paste component is of comparable grain size to the zinc particle grain size. When the paste is mixed with zinc, zinc oxide and electrolyte, the material is formed into a three-dimensional lattice matrix of comparable mesh size to the zinc mesh size, which serves to diminish electrode shape change as well as zinc dendrite formation. The occurrence of zinc ion diffusion is also minimized. The anode paste material contains polymer beads encapsulated by cellulose, and optionally, intermixed with a hydrophobic polymer of hydrogen permeability greater than 1xc3x9710xe2x88x9213 cm3 cmxe2x88x921sxe2x88x921Paxe2x88x921.
The anode paste material contains cellulose as a gelling agent. Cellulose in the form of regenerated cellulose has been a widely used separator for zinc-based batteries. Some of the reasons for this include its low electrical impedance as well as its excellent ion transport in alkaline environments. In the presence of alkaline electrolyte cellulose can swell considerably. As a powder, cellulose is often used as a gelling agent. Cellulose is the principal constituent in the matrix material of the invention. Cellulose, with a degree of polymerization between 200 and 1200, can be used, so long as it is made soluble. This cellulose can also be crosslinked by a variety of methods.
Despite its advantages, cellulose is limited in its ability to accommodate zinc density changes and to transport hydrogen. If cellulose powder acts as the sole gelling agent, zinc ions readily permeate through the gel. Additionally, cellulose possesses one of the lowest hydrogen permeability coefficients of known polymers.
To compensate for these limitations, the anode matrix of the present is a two-component material that incorporates small hydrocarbon beads encapsulated in a cellulose matrix.